New molecules found in comet C/1995 O1 (Hale-Bopp) - Investigating the link between cometary and interstellar material

We present millimetre and submillimetre observations of comet C/1995 O1 (Ha le-Bopp) undertaken near perihelion with the Caltech Submillimeter Observat ory and the 30-m telescope and Plateau-de-Bure interferometer of the Instit ut de Radioastronomie Millimetrique. From a spectral molecular survey, six new cometary molecular species have been identified for the first time in a comet: SO, SO2, HC3N, NH2CHO, HCOOH, and HCOOCH3. Relative abundances with respect to water are 0.3% (SO), 0.2% (SO2), 0.02% (HC3N), 0.01-0.02% (NH2C HO), 0.09% (HCOOH), and 0.08% (HCOOCH3). Several rotational transitions of OCS and HNCO, whose first identifications were made previously in comet C/1 996 B2 (Hyakutake), have also been detected, confirming that these molecula r species are ubiquitous compounds of cometary atmospheres. Inferred abunda nces of OCS and HNCO relative to water in comet Hale-Bopp are 0.4% and 0.1% , respectively. During this observational campaign, we also observed rotati onal lines of HCN, HNC, CH3CN, CO, CH3OH, H2CO, H2S, and CS. In com binatio n with results of other observations, a comprehensive view of the volatile composition of the coma of comet Hale-Bopp is obtained. A quantitative comp arison shows that chemical abundances in comet Hale-Bopp parallel those inf erred in interstellar ices,hot molecular cores and bipolar flows around pro tostars. This suggests that the processes at work in the interstellar mediu m, in particular grain surface chemistry, played a major role in the format ion of cometary ices. It supports models in which cometary volatiles formed in the interstellar medium and suffered little processing in the Solar Neb ula.